Foamed resin injection molding techniques typically employ a molten resin with dissolved gas injected into a mold cavity such that the resin foams in the mold cavity. The foaming of the resin is generally produced by using a gas that is provided in the resin under pressure and the resin is injected into the mold cavity such that the drop in pressure triggers cell nucleation and foaming. The injection of resin with a gas in its supercritical state typically creates micron-sized voids in the thermoplastic polymer upon foam molding, and thus is also referred to as microcellular injection molding. Conventional microcellular injection techniques often produced molded parts that contain surface defects, such as swirls, splays, knit lines and flow lines due to gas diffusion. Hence, conventional techniques generally have limited applications to parts that do not have aesthetic appearance requirements.
Accordingly, it is therefore desirable to provide for a microcellular injection molding apparatus and method that produces molded parts having good aesthetic appearances substantially free of unacceptable surface defects.